Pressure-sensitive adhesives (PSAs) possess properties including at least: (1) aggressive and permanent tack, (2) adherence to an adherend with no more than finger pressure, (3) sufficient ability to hold onto the adherend, and optionally (4) sufficient cohesive strength to be removed cleanly from the adherend.
Hot melt pressure-sensitive adhesives (hereinafter HMPSAs) are widely used in industry. In particular, acrylic HMPSAs are often used in applications where color and/or aging characteristics are important. Typically, acrylic HMPSAs are crosslinked in order to provide desirable shear properties, and various techniques have been used.
One method of crosslinking acrylic HMPSAs involves using irradiation (UV or electron beam); however, such methods generally require significant processing time and specialized equipment. Furthermore, the presence of tackifiers and/or plasticizers in the adhesive formulation can greatly reduce the efficiency of radiation-induced crosslinking.
Another method of crosslinking acrylic HMPSAs is to use metal ions to form ionic crosslinks with the carboxylic acid groups in the polymer chains. The mechanism of ionic crosslinking is thought to be the result of an ionomeric effect. Ionomers are polymers that consist of nonpolar monomers and also ion-containing monomers. Since the nonpolar segments and the polar ionic groups of the polymer are energetically incompatible, microphase separation occurs to produce ion-rich domains (for example, see B. P. Grady, Polymer Engineering and Science, June 2008, vol. 48, No. 6, pp. 1029-1051). The morphology of ion-rich domains dispersed within a softer, nonpolar bulk phase can lead to physical crosslinking, similar to that of block copolymers. Ideally, a pressure-sensitive adhesive ionomer would have sufficiently low viscosity at high temperatures to facilitate coating, and high viscosity at lower temperatures to provide good shear properties.
To prepare an ionically crosslinked HMPSA, a metallic crosslinking agent is typically mixed with a polymer having anionic functional groups. To accomplish this, a solution route is usually employed, whereby metal salts are dissolved into a solution, then added to a polymer solution, followed by removal of the solvent. This solution method requires the use of organic solvents, which may be costly and may raise handling and/or disposal issues.